Whats the round tapped washer with square cutouts for on the BB

[IT_Dummy]

Excuse my technical terms
On the bottom Bracket and the left side there is a thin washer nut (with square cutouts)
It was only hand tight. I am just wondering if has allowed something else to go loose after I have tightened it. What is it for ? It looks like you can adjust some internal part
Help would be most appreciated

 

[RecordAceFromNew]

Think you are talking about the lock nut with slots for the c spanner? If so see this.

 

[PpPete]

It's a locknut. When it's tight it holds the BB cup in a fixed position with respect to the frame and the rest of the BB.
It can be tricky to adjust that cup to just the right degree of tightness and tehn nip up the locknut tight enough without moving the cup.
Alternatively whip the whole lot (remember the r/h cup has a l/h thread on many bikes) and replace with one of these, whcih have no locknut and no need of adjustment.
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EDIT cross posted with RAFN .... again!